1. Field of the Invention
This invention relates to a twist drill and method for producing a twist drill which method includes forming a flute of a twist drill.
2. Background Information
On a conventional bit for a twist drill, the generally two main cutting edges are curved via the chisel edge so that they run into one another in the shape of an “S”. Adjacent to each of the main cutting edges is a main clearance face which transitions into a flute which is realized so that it runs in a spiral shape in the longitudinal direction of the drill. The secondary cutting edge is formed on one of the peripheral sides of the respective flute and transitions into the main cutting edge, forming a face edge. The secondary cutting edge is therefore the cutting edge that extends in a spiral shape in the longitudinal direction of the drill along the respective flute. The term “bit” is used here to designate a longitudinal area of the drill which begins at the end-cutting edge (main cutting edge and chisel edge) and has a length which equals approximately twice the diameter of the drill.
The secondary cutting angle, as defined above, indicates appropriately the geometric orientation of a wedge that forms the secondary cutting edge with reference to the radial direction, i.e. the direction perpendicular to the longitudinal axis of the drill. The term “positive secondary cutting angle”, i.e. a cutting angle of more than 0°, is therefore used when the wedge forms an acute angle. On conventional bits with the curved realization of the main cutting edges and the chisel edge, there is a positive secondary cutting angle. This secondary cutting angle extends with a constant value over the entire cutting length of the drill.
The positive secondary cutting angle has the advantage that the contact area between the chips removed from the workpiece during the drilling process and the wall of the boring in the workpiece is minimized. The chips are therefore discharged very rapidly. In the vicinity of the main cutting edges, however, the positive secondary cutting angle has the disadvantage that the cutting wedge is comparatively weak in the vicinity of the face edge on account of its acute-angle geometry. However, precisely in the vicinity of the face edge is where very high forces occur during drilling. The load is further increased by the fact that during drilling, the face edge digs into the material, so to speak. Thus at this point in particular there is a danger that the drill will break off. A further disadvantage is that the chip thrown off is curved, which requires an additional exertion of force and thus additional cutting efficiency.
To avoid these problems of the positive secondary cutting angle in the vicinity of the major cutting edges, one possibility is to provide a secondary cutting angle with a value of 0°. In this case, therefore, the main cutting edges run in a straight line toward the chisel edges. Consequently the cutting wedge is relatively massive in the vicinity of the cutting face and is therefore stable. The risk of an overload in this area is therefore kept low. A secondary cutting angle of 0°, however, has the significant disadvantage that the chip will not be removed from the boring wall quickly enough over the cutting length of the drill. Under some conditions, this leads to an undesirable surface roughness of the boring wall. For the realization of a secondary cutting angle of 0°, an expensive shaped grinding wheel with a complex geometry is required during the grinding of the drill. On the other hand, with a positive secondary cutting angle, a standard grinding wheel with a simple geometry can be used, which is relatively advantageous.